Blender CUDA Flatpack
You need to install NVIDIA drivers separately via flatpak to enable GPU rendering.

Run  flatpak list  in console.
Check if you have a package with application id similar to org.freedesktop.Platform.GL.nvidia
If not run  flatpak install org.freedesktop.Platform.GL.nvidia 
and install the latest drivers.At the time of writing this the latest driver was org.freedesktop.Platform.GL.nvidia-460-27-04